In situ-formed debond layer for fibers

ABSTRACT

A debonding layer is formed on fibers such as silicon carbide fibers by forming a thin film of a metal such as nickel or iron on the silicon carbide fibers and then annealing at a temperature of about 350-550° C. to form a debond layer of a metal silicide and carbon. These fibers having the debond coating can be added to composite forming materials and the mixture treated to form a consolidated composite. A one heating-step method to form a consolidated composite involves inserting the silicon carbide fibers with just the initial metal film coating into the composite forming materials and then heating the mixture to form the debond coating in situ on the fibers and to form the consolidated composite. Preferred heating techniques include high temperature annealing, hot-pressing, or hot isostatic pressing (HIP).

This is a division of U.S. application Ser. No. 08/831,282, filed onMar. 31, 1997, now U.S. Pat. No. 6,056,907.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an in situ method for producing debond coatingon silicon carbide fibers to be incorporated in a composite and theresulting product.

2. Description of the Related Art

Current debond coatings are usually applied to fibers by chemical vapordeposition (CVD) or chemical vapor infiltration (CVI). These techniquesare generally slow and costly. High temperatures, special atmospheres,special chemicals and special apparatus increase costs. Coatingssometimes contain defects due to touching fibers and incompletespreading of fiber tows. Typical debond layer materials are carbon suchas described by R. C. Loszewski in U.S. Pat. No. 5,024,889 and by H.Sakamoto in U.S. Pat. No. 5,055,430 and boron nitride as described by T.L. Jessen in U.S. Pat. No. 5,407,740 and R. Rice in U.S. Pat. No.4,642,271. Special handling is generally used to prevent damage to thedebond coatings. The ability to place a nickel coating on a fiber isshown by L. G. Morin in U.S. Pat. No. 4,942,090 on carbon fibers.

OBJECTS OF THE INVENTION

It is an object of this invention to control bonding between the fiberreinforcements and the matrix in composite materials.

It is a further object of this invention to improve composite propertiesby providing a debonding layer on fibers in ceramic matrix composites.

It is a further object of this invention to provide tailored debondlayers which absorb energy and make for tougher composite materials.

It is a further object of this invention to provide improved, in situproduced debond coatings on silicon carbide fibers which may be used incomposites to produce superior products.

It is a further object of this invention to provide debond coatings onsilicon carbide fibers where there is an outer layer containing nickelsilicide, Ni₂Si, and carbon.

It is a further object of this invention to provide composite precursorswhere the composite forming material contains silicon carbide fiberswith improved debond coatings.

These and further objects of the invention will become apparent as thedescription of the invention proceeds.

SUMMARY OF THE INVENTION

A debonding coating is formed on fibers, and especially silicon carbidefibers, by forming a thin film of a metal or metal silicon alloy onsilicon carbide fibers and annealing the fibers at a temperaturesufficient to produce an effective amount of a metal silicide and carbonto form a debond coating on the silicon carbide fibers. These fibershaving the debond coating can be added to composite forming materialsand the mixture can be treated to form a consolidated composite. A morepreferred way to make the consolidated composite is to place the siliconcarbide fibers with the initial metal film coating into the compositeforming materials and then to heat the mixture to form the debondcoating in situ on the fibers and to form the consolidated composite.Preferred heating techniques include high temperature annealing,hot-pressing, or hot isostatic pressing (HIP).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(d) presents schematic cross-sectional views of the siliconcarbide fiber and the possible layers in the reaction products.

FIG. 2 is a process flow sheet illustrating the two embodiments of theprocess invention.

FIG. 3 is differential scanning calorimetry heating trace (DSC) for theNi/SiC multilayer sample.

FIG. 4 is an X-ray diffraction scan (XRD) showing that the reactionproduct formed is Ni₂Si.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, interphase modifications are required to control bondingbetween the fiber reinforcements and the matrix in composite materials.For ceramic matrix composites a debonding layer is beneficial forimproving composite properties. Tailored debond layers promote multiplecrack initiations, and crack deflection or branching during propagation,all of which absorb energy and make for tougher composite materials.Improved, in situ produced debond coatings have been formulated whichmay be used in composites to produce superior products.

An in situ-formed debond coating may be synthesized by the followingsequence. Using standard, low cost plating techniques, a thin film ofnickel may be solution plated by either electrodeposition or electrolessdeposition on fibers in a SiC tow such as Nicalon fibers. A chemicalcoating technique, such as the two mentioned above, can penetrate thetow so that fewer missed or bare spots are anticipated. Further, themetal layer should be flexible so that spallation of the coating isminimized.

Processing of the fibers into a composite-would then proceed as normalsuch as with layup, matrix infiltration or coating, and consolidation.During the final composite processing steps, a high temperature anneal,hot-press, or hot isostatic pressing (HIP), would cause the nickel layerto react with the outermost portion of the SiC fiber and form nickelsilicide, Ni₂Si. Pressure is not required for the Ni+Si reaction toproceed. However, pressure is generally useful to consolidate the matrixphase of coated fibers.

Carbon must be released for this reaction to proceed since the reactionis:

4Ni+2SiC→2Ni₂Si+2C

Depending on the heat treatment, the carbon will either form smallpockets or a complete layer in the interphase of the composite. Thepreferred temperature for carrying out the reaction is from about 350°C. to about 550° C.

FIG. 1(a) illustrates the cross-section of a silicon carbide fiber thathas a thin coating of Ni prior to reaction.

The amount of nickel deposited will control the amount of carbonreleased to form the debond layer.

FIGS. 1(b)-(d) show three potential scenarios for the debond layersafter reaction. The resultant debond coating morphology will depend onthe various surface and volume energies of the fiber, reaction products,and matrix as well as diffusion rates. In FIG. 1(b), the SiC fiber has afirst inner layer of Ni₂Si and an outer layer of carbon. In FIG. 1(c),the SiC fiber has an outer layer of a mechanically-weak mixture ofcarbon and Ni₂Si. In FIG. 1(d), the SiC fiber has a first inner layer ofcarbon and an outer layer of Ni₂Si. There is some evidence for thecarbon collecting at the free surface (as in FIG. 1(b)) when the Ni andSiC are reacted isolated from a matrix. The short times and lowtemperatures involved in the reaction due to its large free energychange constrain the diffusion distance the carbon could achieve. Thus,the carbon reaction product is near the interface at all times. At theinterface the carbon will perform its debonding function independent ofwhich resultant morphology is produced. In either event, a weakmechanical bond or debonding layer is established between the remainingfiber and matrix. Clean deposition of a thin nickel layer will be themost important factor for quality control. When the nickel coating isapplied, it is a layer. Once reacted, and especially if the mixturemorphology occurs, the debond coating may not always be a “layer”anymore. It remains a coating, however, since it envelopes the fiber.

The advantages of this direct technique are that the composite is easierand cheaper to fabricate. Due to more complete penetration of the fibertow, the composite will have a better, more reliable performance duringprocessing. The in situ formation of the debond layer means there is noextra processing steps required. The nickel-coated fibers also provide ahigh thermal conductivity path during heating. This process is shown onthe left side of FIG. 2.

An alternative procedure to make the composite is to form the debondlayer at any time by heat treating the nickel-coated SiC fibers. Thus,if desirable, the debond layer could be formed prior to compositeprocessing as shown on the right side of FIG. 4. This procedure,however, introduces an additional heating step.

The fibers with the pre-formed debond coating have to be embedded in amatrix to perform their debonding function. Conceivably, this could bedone without heat, such as in the case of a cured polymer matrix. It isbelieved that all ceramic matrices would require heating. It is assumedin these processes that the chemical reactions caused by the heatingwill not destroy the carbon produced by the first reaction. For example,one would not want to put the fibers having the SiC/Ni₂Si/C combinationinto an aluminum matrix because the Al and C would react to formaluminum carbide and there would no longer be a debond layer.

Other metals besides nickel may be used in the practice of theinvention. As described above with respect to nickel, the function ofthe the metal thin film is to react with silicon carbide at theannealing temperature to form a debond coating of a metal silicide andcarbon. Suitable metals in the invention are any metals that react withsilicon carbide to form a metal silicide and carbon as reactionproducts. Examples of other suitable metals include iron, copper,cobalt, tin, paladium, platinum and mixtures and alloys thereof. Forexample, an Fe layer would perform in essentially the same manner as anickel layer. However, nickel is preferred over iron because it would bedifficult to electro-deposit the Fe as easily as the nickel, andoxidation of the Fe may become a problem. A metal silicon alloy may alsobe used, though it is less preferable.

The two composite making processes result in products which may be inthe same product if all the same materials were used in each case.However, some systems might be amenable to (1) the process where themetal coated fibers are placed in the composite and the consolidatedcomposite is formed by heating with the simultaneous in situ-formeddebond coating on the fibers or to (2) the process where the insitu-formed debond coating is first formed on the fibers and then thesecoated fibers are added to the composite, but not to both.

Having described the basic aspects of the invention, the followingexample is given to illustrate a specific embodiment thereof.

EXAMPLE 1

This example illustrates the reaction which takes place between theadjacent Ni film and the silicon carbide.

A Ni/SiC multilayer sample was synthesized as an analog of afiber/matrix composite. The multilayer sample consisted of alternatingNi and SiC layers of equal thickness of 40 nm so that the bilayer pairwas 80 nm. The total sample was made of 25 bilayers with a thickness ofapproximately 2,000 nm which is 2 microns. The multiple reactioninterfaces mimic the numerous coated fiber interfaces found incomposites and allows for easier determination of the reaction productsvia differential scanning calorimetry (DSC) and X-ray diffraction (XRD).

FIG. 3 is the DSC output at a heating rate of 20° C. per minute for theNi/SiC sample. It indicates a heat release and thus a chemical reactionwas occurring. Samples which had been heat treated to specifictemperatures and quenched were examined for-reaction products by XRD. Atemperature of 410° C. represents a temperature before the major peak inthe DSC scan and a temperature of 510° C. represents a temperature afterthe major peak in the DSC scan.

The XRD plot in FIG. 4 shows conclusively that one of the reactionproducts formed after the DSC scan to 510° C. was Ni₂Si. The vertical,downward pointing arrows indicate the positions of expected x-raydiffraction peaks based upon previously published industry standardpowder diffraction files. The number of the file or “card” is shown. Thefact that three or more arrows line up establish that Ni₂Si was presentin the sample under analysis.

The carbon which is a by-product of the reaction, while not indicated inthe XRD plot, must still be present due to mass conservation. Carbon isnot easily observed by x-rays because carbon, being a low-Z element,does not scatter x-rays very effectively. The short times and lowtemperatures involved in the reaction constrain the diffusion distancethe carbon could achieve. Thus, the carbon is inferred to be near theinterface. At the interface the carbon will perform its debondingfunction. The higher modulus of the Ni₂Si is believed to make it veryuseful for debonding.

It is understood that the foregoing detailed description is given merelyby way of illustration and that many variations may be made thereinwithout departing from the spirit of this invention.

What is claimed is:
 1. A composite product containing silicon carbidefibers having a debond coating made by a process comprising the stepsof: a) forming a thin film of a metal or metal silicon alloy on siliconcarbide fibers, wherein the metal or metal silicon alloy is selected asbeing capable of reacting with silicon carbide to form a metal silicideand carbon; b) adding the coated fibers made in step (a) to a composite;and c) heating the composite at a temperature sufficient to produce aneffective amount of a metal silicide and carbon to form a debond coatingon the silicon carbide fibers at or near the interface of the siliconcarbide fibers in the composite.
 2. A composite product containingsilicon carbide fibers having a debond coating made by a processcomprising the steps of: a) forming a thin film of a metal or metalsilicon alloy on silicon carbide fibers, wherein the metal or metalsilicon alloy is selected as being capable of reacting with siliconcarbide to form a metal silicide and carbon; b) annealing the fibers ata temperature sufficient to produce an effective amount of a metalsilicide and carbon to form a debond coating on the silicon carbidefibers; c) adding the fibers of step (b) to a composite; and d) treatingthe composite of step (c) to form a consolidated composite.
 3. A siliconcarbide fiber with a debonding coating of a metal silicide and carbonmade by a process comprising the steps of a) forming a thin film of ametal or metal silicon alloy on silicon carbide fibers, wherein themetal or metal silicon alloy is selected as being capable of reactingwith silicon carbide to form a metal silicide and carbon; and b)annealing the fibers at a temperature sufficient to produce an effectiveamount of a metal silicide and carbon to form a debond coating on thesilicon carbide fibers.
 4. A silicon carbide fiber according to claim 3,wherein the silicon carbide fiber has a first inner coating of metalsilicide and an outer coating of carbon.
 5. A silicon carbide fiberaccording to claim 3, wherein the silicon carbide fiber has an outercoating of a mechanically-weak mixture of carbon and metal silicide. 6.A silicon carbide fiber according to claim 3, wherein the siliconcarbide fiber has a first inner coating of carbon and an outer coatingof metal silicide.
 7. A composite precursor of silicon carbide fibershaving a thin film of metal or metal silicon alloy or mixtures thereofon the fibers and a composite forming material, said precursor beingmade by the process of a) forming a thin film of a metal or metalsilicon alloy on silicon carbide fibers; and b) adding the coated fibersmade in step (a) to a composite; wherein upon heating said compositeprecursor it forms a composite having a debond coating of a metalsilicide and carbon at or near the interface of the silicon carbidefibers in the composite.
 8. A composite precursor of a composite formingmaterial containing silicon carbide fibers having a debond coating of ametal silicide and carbon on the silicon carbide fibers, said precursorbeing made by the process of a) forming a thin film of a metal or metalsilicon alloy on silicon carbide fibers; b) annealing the fibers at atemperature sufficient to produce an effective amount of a metalsilicide and carbon to form a debond coating on the silicon carbidefibers and c) adding the fibers of step (b) to a composite, wherein uponheating said composite precursor it forms a composite having a debondcoating of a metal silicide and carbon at or near the interface of thesilicon carbide fibers in the composite.